The invention relates to a lean combustion method for a reciprocating internal combustion engine having a cylinder, in which a combustion chamber is delimited by a cylinder head and a piston that can be moved in a reciprocating manner, the piston being able to vary the volume of the combustion chamber, and wherein a fuel can be introduced directly into the combustion chamber. At least one charge cycle intake valve and one charge cycle exhaust valve are provided for a charge cycle. An intermediate compression can be set in the combustion chamber at a charge cycle top dead center, and a main compression can be set at an ignition top center, after an expansion phase.
In the case of HCCI (homogeneous charge compression ignition) methods and CAI (controlled auto ignition) methods, which are in the development stage, the start of the ignition (auto-ignition) is controlled by way of the temperature by the amount of the exhaust gas recirculation rate (AGR). The necessarily high exhaust gas recirculation rate is set, for example, by an early closing of the charge cycle exhaust valves, i.e. before the charge cycle top dead center (LWOT). This leads to a negative valve overlap between the charge cycle intake valve and the charge cycle exhaust valve, because the charge cycle intake valve will open only after the charge cycle top dead center (LWOT) and therefore results in an intermediate compression in the charge cycle top dead center (LWOT).
By way of a pre-injection during this intermediate compression, it becomes possible to either raise the temperature (while the injected fuel is partially or completely converted), or intermediate products are formed, which result in an increase of the ignition performance of the mixture. By way of this pre-injection, the start of the auto-ignition can be influenced. Corresponding examples are indicated, for example, in German Patent document DE 198 10 935 C2, on which the present invention is based. DE 198 10 935 C2 basically describes a method of operating a four-stroke reciprocating internal combustion engine with a homogeneous lean basic mixture of air, fuel and retained exhaust gas, as well as with a compression ignition and a direct fuel injection into the combustion chamber, whose volume changes cyclically and which can be filled with intake gas through at least one inlet element, and the combustion exhaust gases can be pushed through at least one outlet element at least partly out of the combustion chamber. In this case, in an activating phase, the retained exhaust gas is compressed in the area of the charge cycle top dead center and subsequently is expanded and that, in this phase of the operating cycle, activation fuel for stabilizing the main combustion is injected into the combustion chamber.
Furthermore, in DE 198 10 935 C2, the pre-injection of fuel is used for the stabilization of the main combustion and expansion of the operating range to low loads. In the case of a complete combustion of the fuel, the thermal energy of the retained exhaust gas is increased for ensuring the combustion of the next cycle. If the combustion is incomplete, at least the chemical activity of the retained amount of exhaust gas is increased (formation of radicals, activation) without clearly increasing the temperature in the process. The amount of fuel converted to activated fuel is to be controlled by way of the point-in-time of the pre-injection.
A distribution of the injected fuel quantity that is as homogeneous as possible is a requirement in this case. In reality, however, at the points-in-time of the injection in the area of the charge cycle top dead center, i.e., at temperatures at which the ignition delay time of fuel is in the required time range, the fuel will already react before a homogeneous distribution of the fuel is present in the combustion chamber. In these cases, the degree of homogenization will vary from one working cycle to the next, and a control of converted to activated fuel will not be possible in reality. This effect is still intensified by temperature fluctuations in the retained exhaust gas and fluctuations in the degree of the charge motion.
Thus, by means of the method described in DE 198 10 935 C2, the desired stabilization of the main combustion by activation is only theoretical but cannot be shown in practice.
It is an object of the present invention to provide a robust method for stabilizing the main combustion that can be implemented in practice.
This and other objects are achieved by a lean combustion method for a four-stroke reciprocating internal combustion engine having a cylinder, in which a combustion chamber is delimited by a cylinder head and a piston that can be moved in a reciprocating manner, the piston being able to vary the volume of the combustion chamber, and wherein a fuel can be introduced directly into the combustion chamber. At least one charge cycle intake valve and one charge cycle exhaust valve are provided for a charge cycle. An intermediate compression can be set in the combustion chamber at a charge cycle top dead center, and a main compression can be set at an ignition top dead center, after an expansion phase. For a controlled conversion of a pilot quantity of fuel to intermediate products (CH2O, H2O2, CO, C2H4, C3H4, etc.) and/or the complete combustion products (CO2, H2O):
(a) the introduction of the pilot quantity of fuel takes place at the latest by the charge cycle top dead center,
(b) a maximal intermediate-compression temperature can be set by varying the closing point-in-time of the charge cycle exhaust valve,
(c) a maximal intermediate-compression temperature can be set by varying the opening point-in-time of the charge cycle intake valve before the charge cycle top dead center LWOT, and/or
(d) a variable compression during the intermediate compression, and/or the mass of the pilot quantity of the fuel, can be varied.
In order to achieve a sufficient homogenization of the pre-injected fuel quantity, the latter has to be injected in the time periods in which the retained exhaust gas has conditions (temperature, pressure, air ratio) which are uncritical in the case of the present ignition delay times; i.e., the fuel reacts very slowly, so that sufficient time remains for the homogenization. This can most easily be achieved by an injection immediately after the charge cycle exhaust valve closes (AS). At that point-in-time, the temperature of the retained exhaust gas will be the lowest, and the time period to the desired reaction will be the longest.
Since, by way of the injection or the jet design of the fuel, no “perfect” homogenization can be carried out, the time from the injection point-in-time of the fuel to the reaction can be utilized for assisting the homogenization by corresponding measures, such as increasing the charge motion or the degree of turbulence. The increase of the degree of turbulence can take place, for example, by a variable valve stroke, by valve stroke phasing (different charge cycle valve stroke courses for equally acting charge cycle valves) on the exhaust side, quench areas in the combustion chamber, etc. These are measures known to a person skilled in the art. The then still existing fluctuations of the degree of homogenization will largely be negligible.
Because, in the case of a control of the activated or converted fuel by way of the point-in-time of the pre-injection will no longer be possible, the following method is provided.
In the case of the given homogenization, the ignition delay time of the fuel essentially depends on the temperature, the pressure and the air ratio in the combustion chamber. This is additionally influenced by the rotational speed of the internal-combustion engine because the “time-related aspects” depend on the rotational speed.
If the dependencies of the ignition delay time on the temperature, the pressure and the air ratio at different rotational speeds are known, according to the invention, the desired quantity of converted or activated fuel can be set and controlled by way of these values.
These quantities can be easily set, for example, by the point-in-time for the closing of the charge cycle exhaust valve (AS), and the pre-injection quantity of fuel.
By means of the method according to the invention, the HCCI operating range can be expanded without any problem while the consumption, NOx emissions and combustion noise are reduced. Furthermore, the method according to the invention is a lean combustion method that can be used worldwide for different fuel qualities. Advantageously, the expenditures and costs for the exhaust gas aftertreatment are reduced because costly lean exhaust gas aftertreatment with high technical expenditures can be eliminated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.